But Obama administration officials distanced themselves from Mr. Comey at the time. They said they had seen no evidence to support the idea of a “Ferguson effect,” named after the 2014 shooting by a police officer of an unarmed black man in Ferguson, Mo., which sparked widespread protests.
Obama administration officials declined to comment on Wednesday about Mr. Comey’s latest remarks, which were sharper in tone than his previous statements. But some dissenters said he was needlessly stirring up an unproven and divisive notion.
“He ought to stick to what he knows,” James O. Pasco Jr., executive director of the National Fraternal Order of Police, said in a telephone interview. The organization has more than 330,000 members.
“He’s basically saying that police officers are afraid to do their jobs with absolutely no proof,” Mr. Pasco said.
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Mr. Comey said that he could not fully explain the trend or the major differences from one city to the next.
“I don’t know what the answer is, but holy cow, do we have a problem,” he said.
“It’s a complicated, hard issue, but the stakes couldn’t be higher. A whole lot of people are dying,” he said.
He said that the spike in violent crime deserves more national attention from scholars, the media, and the public.
“Something is happening,” he said. “A whole lot more people are dying this year than last year, and last year than the year before and I don’t know why for sure.”
Asked about his past views on the “Ferguson effect” as a possible explanation, Mr. Comey said he rejected that particular term, but added that he continued to hear from police officials in private conversations that “lots and lots of police officers” are pulling back from aggressive confrontations with the public because of viral videos.
